The present invention relates to a sliding member comprising a substrate and a coating layer of solid lubricant plate crystal grains adhered to the surface of the substrate and a process for forming the coating layer.
In the plain bearings (sliding members) of engines for automobile, aluminum-based bearing alloys and copper-based bearing alloys are generally used as a sliding layer. In recent years, the engines for automobiles have been increased in engine power and engine speed but have been strongly required to be improved in fuel consumption rate. Therefore, the plain bearings are required to have low frictional properties. Particularly in the case of automobiles in which the start and stop of the engine are often repeated, such as hybrid cars, low frictional properties at boundary lubrication are demanded.
In satisfying the demand for the low frictional properties at boundary lubrication, it is effective to modify the surface of the plain bearing by forming a coating layer of a solid lubricant. For the modification of the bearing surface by the use of the solid lubricant, there are known techniques in which solid lubricant grains are allowed to collide with the plain bearing surface by application of a shot peening or shot blasting technique to be adhered to the bearing surface by the energy of collision (see, for example, patent documents 1 and 2).
Patent document 1: JP-A-11-131257
Patent document 2: JP-A-2002-339083
The above-mentioned patent document 1 discloses that zinc lubricant powder having an average grain size of 100 μm is allowed to collide with a substrate by the use of a blasting apparatus to be diffused and infiltrated into the inside of the substrate. The patent document 2 discloses that a coating layer is formed by allowing molybdenum disulfide with an average grain size of 1 to 20 μm and a purity of not less than 95% to collide with the piston, cylinder bore or the like of an engine at a projection speed of not less than 100 m/s and that an excellent reducing effect on the coefficient of friction can be obtained owing to the coating layer.
However, when solid lubricant grains are allowed to collide with the surface of a substrate to form a coating layer of the solid lubricant, the coating layer can be formed as a layer rich in the solid lubricant grains but the substrate surface is work-hardened by the collision of the solid lubricant grains, so that the conformability is deteriorated, resulting in a deteriorated anti-seizure property. In addition, when solid lubricant grains are allowed to collide with a substrate, a film is formed by the diffusion of the solid lubricant grains into the substrate or by the infiltration of the solid lubricant grains into the substrate by the force of impact and their adhesion to the substrate. Therefore, only a very thin coating layer can be formed and it is early worn away.
The present invention was made in view of the above situation. A first object of the present invention is to provide a sliding member having a low coefficient of friction and an excellent wear resistance. A second object of the present invention is to provide a process for forming the coating layer of a sliding member which makes it possible to form the coating layer rich in a solid lubricant on the surface of a substrate for the sliding member without work-hardening the substrate surface.
Molybdenum disulfide, graphite, tungsten disulfide and boron nitride are often used as solid lubricants. They have the form of plate crystal grains and the plate crystal grains have a layer crystal structure formed by piling mainly layers with (00l) planes in terms of Miller indices as the maximum face in parallel. For example, plate crystal grains of molybdenum disulfide have a structure in which as shown in FIG. 7, layers formed by connecting MoS2 molecules in a direction parallel to the xy-plane are piled in the z-axis direction. Only weak van der Waals force is applied between the layer 1 and the layer 2 which are adjacent to each other. As shown in FIG. 6, such solid lubricant plate crystal grains 3 have the outer shape of a relatively thin plate.
When plate crystal grains having a layer crystal structure are present as a solid lubricant in the coating layer of a sliding member and shearing force is applied between layers in the plate crystal grain with the movement of a counter member, the van der Waals force is easily overcome, so that slide occurs between the layers. This interlaminar slide is characteristic of a substance having a layer crystal structure and the coefficient of friction in this case is very small. This is a mechanism by which the sliding member has low frictional properties when the plate crystal grains are used as a solid lubricant.
The present inventor has accomplished the invention by having the following expectation: since a mechanism by which solid lubricant plate crystal grains having a layer crystal structure have low frictional properties is as described above, still lower frictional properties can be attained when each plane between layers in the solid lubricant plate crystal grain constituting a coating layer is parallel to the direction of movement of a counter member, namely, it is parallel to the surface of the coating layer.
According to the present invention, the following sliding members and processes for producing the coating layer of each of the sliding members are provided.
(1) A sliding member comprising a substrate and a coating layer having a sliding surface and adhered to the surface of the substrate, wherein
said coating layer is formed by laminating solid lubricant plate crystal grains and contains not more than 10 mass % of a resin for binder for bonding the solid lubricant plate crystal grains, and
said solid lubricant plate crystal grains have a crystal structure formed by piling (00l) planes (wherein l is an integer of not less than 1) in parallel, the index of orientation of the (00l) planes being not less than 90% at least on said sliding surface.
(2) A sliding member according to (1), wherein said solid lubricant plate crystal grains are composed of not less than one compound selected from molybdenum disulfide, graphite, tungsten disulfide, boron nitride and molybdenum trioxide.
(3) A sliding member according to (1) or (2), wherein said resin for binder is contained in said coating layer in a proportion of 2 to 7 mass %.
(4) A sliding member according to any one of (1) to (3), wherein said resin for binder is composed of a thermoplastic resin or a thermosetting resin.
(5) A process for forming the coating layer of a sliding member according to any one of (1) to (4), which comprises
subjecting the solid lubricant plate crystal grains having a crystal structure formed by piling (00l) planes in parallel, to free adhesion to adhesion media together with powder of the resin for binder,
sliding the adhesion media having the solid lubricant plate crystal grains and the powder of the resin for binder subjected to the free adhesion to the adhesion media, on the surface of the substrate while applying a pressure to the substrate surface, to adhere the solid lubricant plate crystal grains to the substrate surface with rubbing, together with the powder of the resin for binder so that the (00l) planes of the solid lubricant plate crystal grains may be parallel to the substrate surface,
sliding the above-mentioned adhesion media on the surfaces of the solid lubricant plate crystal grains adhered to the substrate surface, under pressure to adhere the solid lubricant plate crystal grains adhered freely to the adhesion media, to the surfaces of the solid lubricant plate crystal grains adhered to the substrate surface, together with the powder of the resin for binder to effect laminating, and
infiltrating the powder of the resin for binder into the spaces among the above-mentioned solid lubricant plate crystal grains by allowing the powder to assume a semi-molten state or a molten state by heating the substrate and/or generating frictional heat by adjustment of the sliding speed of the adhesion media, at the time of the adhesion of the above-mentioned solid lubricant plate crystal grains and the powder of the resin for binder.
(6) A process for forming the coating layer of a sliding member according to (5), wherein the powder of said resin for binder is composed of powder of a thermosetting resin, and said resin for binder is infiltrated into the spaces among the above-mentioned solid lubricant plate crystal grains and then heated to be cured.